Garry's Mod File Download Help?

Hello. I just downloaded the steam game Garry's Mod, and The Orange Box as a source engine.I looked up an online guide to downloading files for gmod, but nothing is downloading for me. The download says "succesful", but when I log into Garry's Mod, the thing I downloaded doesn't appear. (for instance if I download a weapon, the weapon won't show up in the "weapons" in game file after downloading). Here's what I do: go to garrysmod.org, find a file to download, click download, click save, save to a folder I created called "Gmod Files", after downloading click open, file opens in winrar, click "extract files", extract to desktop. click "program files", "steam","steamapps","MYSTEAMNAME","garry… move extracted file from desktop to "addons" folder. Someone tell me what I'm doing wrong and why what I download won't show up in game.